Primer composition for heat curable silicone rubbers

ABSTRACT

A PRIMER COMPOSITION FOR BONDING A HEAT CURABLE SILICONE RUBBER TO A METAL OR FABRIC SUBSTRATE, SAID COMPOSITION COMPRISING AN ALKENYL ACYLOXYSILANE, AN ORGANIC PEROXIDE, A SILICONE POLYMER SUBSTANTIALLY FREE OF HYDROXYL GROUPS CONTAINING FROM 0.01 TO ABOUT 5.0 MOLE PERCENT OF UNSATURATED HYDROCARBON GROUPS AND A HYDROCARBON OR HALOGENATED HYDROCARBON SOLVENT.

United States Patent 3,769,253 PRIMER COMPOSITION FOR HEAT CURABLESILICONE RUBBERS Joseph Eugene Stevenson, Charleston, S.C., assignor toStauffer Chemical Company No Drawing. Continuation-impart of applicationSer. N 0.

70,196, Sept. 8, 1970, which is a continuation of application Ser. No.716,340, Mar. 27, 1968, both now abandoned. This application Apr. 5,1972, Ser. No. 241,421

Int. Cl. C08c 11/22 US. Cl. 26033.6 SB 4 Claims ABSTRACT OF THEDISCLOSURE A primer composition for bonding a heat curable siliconerubber to a metal or fabric substrate, said composition comprising analkenyl acyloxysilane, an organic peroxide, a silicone polymersubstantially free of hydroxyl groups containing from 0.01 to about 5.0mole percent of unsaturated hydrocarbon groups and a hydrocarbon orhalogenated hydrocarbon solvent.

This application is a continuation-in-part of my copending applicationSer. No. 70,196, filed on Sept. 8, 1970, now abandoned, which was astreamlined continuation of my application Ser. No. 716,340, filed onMar. 27, 1968 and now abandoned.

The present invention relates to primer compositions and moreparticularly to primer compositions for bonding heat curable siliconerubbers to substrates.

Heretofore, silicone rubbers have been bonded under heat and pressure tosubstrates by coating the substrates with various alkyl acyloxyor alkylalkoxysilanes prior to the application of the silicone rubbers. Amongthe silanes which have been used as primers are tetraethylorthosilicates, t-butoxytriethoxysilanes, methyltriethoxysilanes,methyltriacetoxysilanes, ethyltriacetoxysilanes, propyltriacetoxysilanesand the like.

However, the heretofore known primers have a tendency to migrate on thesubstrate surface when pressure is applied in the application of heatcurable rubbers. Also, substrates which have been coated with theconventional primers do not provide good day-to-day reproducibility,especially when stored for long periods of time. In addition to thedisadvantages enumerated above, these primers exhibit an extensivewicking effect.

Therefore, it is an object of this invention to provide unique primercompositions for substrates. Another object of this invention is toprovide primer compositions for heat curable silicone rubbers. Stillanother object of this invention is to provide primer compositions forbonding heat curable silicone rubbers to fabrics. Still another objectof this invention is to provide primer compositions which will notmigrate on the substrate surface when subjected to pressure in theapplication of heat curable silicone rubbers. A further object of thisinvention is to provide primer compositions which are reproducible on aday-to-day basis and are stable even when stored for long periods oftime. A still further object of this invention is to provide primercompositions which are substantially non-wicking and have bettercalenderability. A still further object of this invention is to providean article having heat curable silicone rubbers bonded thereto.

The foregoing objects and others which will become apparent from thefollowing discussion are accomplished in accordance with this invention,generally speaking, by providing compositions which may be used forbonding heat curable silicone rubbers to various substrates. Thecompositions of this invention comprise a silicone polymer solution,catalyst and a silane of the formula Patented Oct. 30, 1973 ice in whichR is an alkenyl radical having from 1 to 10 carbon atoms; R is an alkylgroup having from 1 to 5 carbon atoms; and n is an integer of from 1 to2..

Referring to the above formula, R is an unsaturated hydrocarbon chainwhich may be open or closed, that is to say, it may be unsaturatedstraight-chain aliphatic hydrocarbon or an unsaturated cycloaliphatichydrocarbon. The hydrocarbon will contain at least one double bond andwill contain from about 2 to 10 carbon atoms and more preferably fromabout 2 to 6 carbon atoms. EX- amples of such unsaturated hydrocarbonchains are vinyl, l-propenyl, 2-propenyl, isopropenyl, l-butenyl,Z-butenyl, 3-butenyl, l-isobutenyl, 2-isobutenyl, l-sec-butenyl,2-secbutenyl, I-methyl-Z-propenyl, l-pentenyl, Z-pentenyl, 4- pentenyl,l-hexenyl, 3-hexenyl, 5-hexenyl; corresponding branched-chain isomerssuch as 3,3-dimethyl-1-butenyl, 2,3 dimethyl-l-butenyl, 2,3dimethyl-Z-butenyl, 2,3-dimethyl-3-butenyl, 1 methyl-l-ethyl-2-propenyl,and the various isomers of heptenyl, octenyl, nonenyl, deeenyl,cyclo-hexenyl, bicyclo-heptenyl, cyclo-pentadienyl, butadiene, and thelike. Preferably, R is a vinyl group. :In the foregoing formula, R is analkyl group, preferably a lower alkyl group, containing from 1 to 5carbon atoms, such as methyl, ethyl, propyl, butyl, pentyl, and thelike. Exemplary of the suitable compound is vinyltriacetoxysilane.

A solution of the primer compositions of this invention is prepared bydissolving a silicone polymer in an organic solvent in a concentrationof from about 1 to about 30 percent, and thereafter adding an alkenylacyloxysilane and a curing catalyst with agitation. The primercomposition may be applied to the substrate by spraying, dipping,brushing, and wiping, and in the case of fabrics, impregnated with thesolution and thereafter dried. Preferably, the primer composition isdried prior to the application of the silicone rubber. Generally, thedried primer composition is interposed between the silicone rubber andthe substrate so that when the rubber and substrate are brought togetherand heated under pressure, extremely good bonding between the rubber andthe substrate is affected.

It is preferred that the primer composition be applied in a solventsolution containing from about 1 to 30 percent by weight of the silaneand more preferably from about 2 to 5 percent by weight of the silaneand from 1 to 30 percent by weight of the silicone polymer. Examples ofsuitable solvents which may be used are aromatic hydrocarbons, such asbenzene, toluene, xylene, naphthylene, and the like; halogenatedaromatic hydrocarbons, such as chlorobenzene; aliphatic hydrocarbons,such as pentane, hexane, octane, decane; and halogenated aliphatichydrocarbons, such as methylene chloride, carbon tetrachloride,perchloroethylene, and the like.

The drying rate of the primer compositions are pri marily dependent upontwo factors. One factor is the rate of condensation of the silane orpartial hydrolyzates thereof which is dependent first on the size of theR and R radicals and second, on the amount of water present for thehydrolysis of the R radicals to volatile acids. Where R and R are largerthan the claimed limits, the rate of hydrolysis and condensation of thesilanes is reduced to the point where the primer compositions do not drysatisfactorily. Where the silane is exposed to water as, for example, inthe form of steam, hydrolysis and condensation may be accelerated.

The other factor which may influence the drying rate is the presence orabsence of a solvent. If a solvent is employed, the volatility of thesolvent effects the drying rate of the primer coating. Thus, the solventshould be sufficiently volatile to evaporate at least as rapidly as thesilane can hydrolyze and condense under the coating conditions.

Due to the possible variations of the two factors above, it isimpossible to set limits on the drying time for the coating which mayvary from a few seconds to several hours.

Heat curable silicone rubbers are elastomeric organosilicone polymers,that is to say, organopolysiloxanes which upon heating in combinationwith an appropriate curing catalyst are converted to the solidelastomeric state. Since the heat and pressure employed during thebonding operation of the present invention may be relied upon to convertthe organopolysiloxane to the solid elastomeric state, theorganopolysiloxane actually employed in making this assembly may be andpreferably is in the uncured state, that is to say, convertible state.

The silicone polymers and heat curable silicone rubbers are well knownto those familiar with the art. They may be described as polysiloxaneswhich are substantially free of hydroxyl groups and contain hydrocarbonradicals, silicon, and oxygen atoms. These silicone polymers and rubbershave recurring structural units of the formula:

wherein R, which may be the same or different, are monovalenthydrocarbon groups, such as alkyl, aryl, aralkyl, alkaryl, cyclic, andalkenyl radicals. Examples of suitable alkyl radicals are methyl, ethyl,propyl, butyl, hexyl, etc.; aryl radicals are phenyl, naphthyl, etc.;aralkyl radicals are benzyl, phenethyl, etc.; alkaryl radicals aretolyl, etc.; cyclic radicals are cyclopentyl, cyclohexyl, etc.; andalkenyl radicals, such as vinyl radicals. In addition, R" may representhalogenated monovalent hydrocarbon groups, such as fluoromethyl,fiuorophenyl, tetrafluoroethyl, trifluorovinyl, trifiuorotolyl,hexafluoroxylyl, heptafluoropropyl, and the like. Preferably, the R"(s)are the same and are lower alkyl radicals. The silicone polymers areused in the primer composition in an uncured state, that is to say,convertible state.

In the preferred embodiment, the polysiloxanes have an average ratio ofhydrocarbon groups to silicon atoms of from about 1.9 to about 2.0hydrocarbon groups and more preferably the polysiloxane contains anaverage of from 1.98 to 2.0 hydrocarbon groups per silicon atom. It ispreferred that these polysiloxanes contain from about 0.01 to 5.0 molpercent and more preferably from 0.05 to 2.0 mol percent of siliconbonded unsaturated hydrocarbon groups such as vinyl, allyl and the like.Examples of suitable silicone polymers and heat curable silicone rubbersare methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber,and the like. The specific silicone polymer or rubber employed inaccordance with the present invention is not critical and may beselected from any of the well-known varieties.

As is conventional, the silicone rubbers are mixed with curingcatalysts, such as benzoyl peroxide, di-t-butyl peroxide,2,4-dichlorobenzoyl peroxide, 2,5 bis(t-butylperoxy)-2,5-dimethylhexane,and a filler, if desired, and then applied to the primed substrate.

Any of the conventional fillers may be incorporated in the siliconerubber. Among the fillers which may be employed are fumed andprecipitated silicas, such as Cab-O- Sil, QUSO, Hi-Sil, Santocel,diatomaceous earths, clays, lithopone, ferric oxide, titanium dioxide,talc, zinc oxide, various forms of carbon, etc. The filler may beincorporated in amounts ranging from about to 90 percent, preferablyfrom about to 75 percent of the total weight of the silicone rubber andthe filler.

The primer composition may be applied directly to greige goods, althoughit may be beneficial to treat a sized fabric with heat and/or a solventprior to the application of the primer. Following the removal of thesizing agent, the fabric may be dip coated or otherwise impregnated withthe primer composition and dried at sufiicient temperature and humidityto volatize off the solvent and cause hydrolysis and condensation of thesilane. After the primer composition is dried, the silicone rubber maybe applied to the coated fabric.

In another embodiment of this invention, it is preferred thatfluorinated silicone polymers be used in the primer compositions whereit is desirable to promote adhesion of fiuorinated silicone rubbers tometal substrates. Before the primer compositions are applied to metalsubstrates, they should be clean and free from loose oxide scale. Thismay be accomplished by several methods known to those skilled in theart. For example, the surface may be first degreased as by dipping themetal substrates in a degreasing solution or by subjecting thesubstrates to vaporized degreasing materials, such as trichloroethylene.Following the degreasing operation, they may be further cleaned byscouring, heating or by blasting the surface with conventionalmaterials, such as steel, shot, grit, sand, and the like.

In bonding silicone rubbers to the primed substrates, the temperatureemployed during the bonding operation may vary somewhat depending uponthe nature of the silicone rubbers, the type of catalysts, and thesubstrates to which they are being bonded. In the case of fabrics,generally temperatures ranging from about to about 300 C. may beemployed. However, sufiicient time should be allowed to cure the rubberand to provide for the bonding of the silicone rubbers to thesubstrates, which may range from about 1 minute to about 48 hours,including any postcure. In general, the coated fabric is press cured forabout 10 minutes at a temperature of about 116 C. and then postcured foran additional 4 hours at a temperautre of about 205 C. The pressureemployed may also vary and may be as low as that merely required toprovide intimate contact, that is, a few pounds per square inch. Theupper limit of pressure is not critical and pressures as high as a fewthousand pounds per square inch may be used.

The bond provided by the press cure may be further improved byadditional curing at higher temperatures for longer periods of time. Forexample, the assembly may be heated for several hours at a temperatureof from about to about 300 C. by placing it in an oven through which hotair is circulated.

Substrates may be coated with the primer compositions of this inventionand stored for up to 3 months and thereafter coated with the siliconerubber in the conventional manner.

In using the primer compositions of this invention, it has been foundthat silicone rubbers and flourinated silicone rubbers may be bonded toa wide variety of substrates, such as metals and fabrics containingsynthetic fibers, such as nylon, Nomex, asbestos, Dacron, as well asglass fibers.

The primer compositions of this invention provide a protective coatingfor the substrates, which are not effected by boiling water even in thepresence of detergents; resist abrasion, and are highly resistant toheat. In addition, the primer compositions do not migrate on the surfaceof the substrate when silicone rubbers are applied under pressure, thusproviding for greater adhesion between the silicone rubbers and thesubstrates.

Various aspects of the invention are further illustrated by thefollowing examples in which all parts are by weight unless otherwisespecified.

EXAMPLE 1 A primer composition is prepared by adding 5 parts of methylvinyl silicone polymer to about 92 parts of toluene and mixed untilsmooth. About 4 parts of vinyltriacetoxysilane and about 0.18 part ofbenzoyl peroxide powder are added to the solution and mixed until auniform dispersion is formed. A fabric, previously treated with acetoneand dried, is dipped in the dispersion and then dried TABLE I Bondstrength (p.p.l.)

Press cured, Postcure,

min 4 hrs./ Substrate 116 0. 205 0.

Nylon. 22(c) 27(c) Nomex 18(0) 21(0) Dacron... 8(a) (c) Asbestos. (a)(a/c) Glass fiber 16 (a) (c) 1 Du Pont trademark.

(a) Adhesive failure.

(0) Cohesive failure.

(a/c) Adhesive and cohesive failure.

EXAMPLE 2 (a) A solution of the primer composition is prepared by adding10 parts of methyl vinyl silicone polymer to about 87 parts of tolueneand mixed until smooth. About 4 parts of vinyltriacetoxysilane and about0.36 part of benzoyl peroxide powder are added to the solution and mixeduntil a uniform dispersion is formed. A fabric, previously treated withacetone and dried, is dipped in the dispersion and then dried at about30 C. at a relative humidity of at least 50 percent for about 1 hour.

A methyl vinyl silicone rubber stock catalyzed with 2,4-dichlorobenzoylperoxide is applied to the coated fabric and press cured for about 10minutes at 116 C. and then postcured for an additional 4 hours at about205 C. The bond between the silicone rubber and the fabric is measuredby the conventional stripping test, ASTM D42958, Method B Modified 180.The results illustrated in Table 11 show a cohesive failure of thesilicone rubber in a majority of the tests.

TABLE II Bond strength (p.p.i.)

Press cured, Posteured,

10 min 4 hrs./ Substrate 116 0. 205 0 Nylon 33(e) 35(0) Nomex 1 22(c)30(e) Dacron (e) (c) Asbestos 17 (a) 21 (a/c) Glass fiber 25(0) (0) DuPont trademark. N o'rE.See footnotes (a) (c) (a/c) bottom of Table I.

(b) A primer composition prepared in accordance with the proceduredescribed above is applied to several previously degreased and driedmetal substrates and dried for about 1 hour at C. in a relative humidityof TABLE III Press cured, Postcured,

10 min./ 4 hrs./ Substrate 6 0. 205 0 Steel 28 (c) Stainless-steel 29Copper 15 (a) Brass. a) Aluminum. 24(0) 27 (0) Titanium 29(0) 24(0)Norm-See footnotes (a) (e) (ale) bottom of Table I.

(c) In accordance with the procedure described in Example 2(a) above,trifluorovinyl silicone polymers are substituted for the methyl vinylsilicone polymer. The primer composition is applied and dried onpreviously degreased metal substrates. A trifluorovinyl silicone rubberstock catalyzed with 2,4-dichlorobenzoyl peroxide is applied to theprimed substances and press cured for 10 minutes at 116 C. and thenpostcured for 24 hours at 249 C. The bond strength between the siliconerubber and the metal substrate is illustrated in Table IV (qualitativetest).

TABLE IV Bond Strength (p.p.i.)

Press cured, Postcured,

10 min 4 hrs Substrate 116 0 205 0.

Steel (c; (0; Stainless-steel (e (0 Copper (c) (a) B (a) No'rE.-Seefootnotes (a) (c) bottom of Table I.

EXAMPLE 3 A solution of the primer composition is prepared by adding 15parts of methyl vinyl silicone polymer to about 82 parts of toluene andmixed until smooth. About 4 parts of vinyltriacetoxysilane and about0.54 part of benzoyl peroxide powder are added to the solution and mixeduntil a uniform dispersion is formed. Greige goods are dipped in thedispersion and then dried at about 30 C. at a relative humidity of atleast 50 percent for about 1 hour.

A methyl vinyl silicone rubber stock catalyzed with 2,4-dichlorobenzoylperoxide is applied to the coated substrate, press cured for about 10minutes at 116 C. and then postcured for an additional 4 hours at about205 C. The bond between the silicone rubber and the substrate ismeasured by the conventional stripping test, ASTM D429-58, Method BModified 180. The results shown in Table V illustrate cohesive failureof the silicone rubber in a majority of the tests.

TABLE V Bond Strength (p.p.i.)

Press cured, Postcured, 10 min 4 hrs./ Substrate 6 0 205 0.

36(0) 32(0) 25(c) 20( /c) Glass fiber 25(c) (c) 1 DuPont trademark.Norm-See footnotes (a) (c) (a/c) bottom of Table 1.

EXAMPLE 4 TABLE VI Bond Strength (p.p.l.)

Press cured, Postcured,

10 min./ 4 hrs./

Substrate 116 0. 5 C

12(a) 8(a) 4(8) 9(8) 12(a) 1 DuPont trademark. (a) Adhesive failure.

It is obvious from a comparison of the above examples that a primercomposition comprising a silicone polymer, an alkenyl acyloxysilane andcatalyst provides for improved adhesion when compared with a primarcomposition consisting solely of an alkenyl acyloxysilane.

When the above examples are repeated in whichisopropenyltriacetoxysilane, 1-hexenyltriacetoxysilane, 1-octenyltriacetoxysilane, vinyltripropionyloxysilane, andisopropenyltrivaleryloxysilane are substituted for vinyltriacetoxysilanein combination with other heat curable silicone rubbers, such as methylphenyl vinyl silicone rubber, the adhesion between the fabrics and thesilicone rubbers are equally as good as those of the specified examples.

Although specific examples of the invention have been described herein,other variations and modifications falling within the spirit and scopeof the appended claims are to be included therein.

The invention claimed is:

1. A primer solution for bonding heat curable silicone rubbercompositions to substrates consisting essentially of a silicone polymerwhich is substantially free .of hydroxyl groups and consists ofhydrocarbon radicals, halogenated hydrocarbon radicals, silicon andoxygen atoms and has recurring structural units of the formula R" SiO,in which the ratio of R" groups to Si atoms is from 1.9 to 2.0 and from0.01 to about 5.0 mol percent of the R" groups are unsaturated and theremaining R" groups are selected from the class consisting ofhydrocarbon and halogenated hydrocarbon groups, an organic peroxidecuring catalyst, a silane of the formula:

wherein R is an alkenyl radical having from 1 to 10 carbon atoms; R isan alkyl radical having from 1 to 5 carbon atoms; and n is an integer offrom 1 to 2 and an inert organic solvent selected from the classconsisting of aliphatic, aromatic and halogenated aliphatic and aromatichydrocarbons.

2. The composition of claim 1 wherein the silane and the siliconepolymer are each present in an amount of from 1 to 30 percent by weightbased on the weight of the composition.

3. The composition of claim 1 wherein the silicone polymer containsrecurring structural units of the formula:

in which 0.05 mol percent of the R" groups are vinyl radicals and theremaining R" groups are selected from the group consisting ofhydrocarbon radicals and fluorinated hydrocarbon radicals.

4. The compositionof claim 1 wherein R is a vinyl group and R is amethyl group.

References Cited UNITED STATES PATENTS 3,264,258 8/ 1966 Ceyzeriat260--46.5 3,474,064 10/1969 Hittmair et a1. 260-37 3,325,449 6/1967Ceyzeriat et al 260-46.5

DONALD E. CZAJA, Primary Examiner M. I. MARQUIS, Assistant Examiner US.Cl. X.R.

161--l93, 206, 207, 213, 227; 26033.8 SB, 37 SB, 46.5 G l

